dc-grid physical modeling platform design and …

DC-GRID PHYSICAL MODELING PLATFORM DESIGN AND

SIMULATION*

Minxiao Han1, Xiaoling Su**

1, Xiao Chen

1, Wenli Yan

1, Zhengkui Zhao

2

1State Key Laboratory of Alternate Electrical Power System with Renewable Energy

Sources, North China Electric Power University

Beijing, China 2State Grid Qinghai Electric Power Maintenance Company,

Qinghai, China

ABSTRACT

This work develops a 6-terminal low voltage DC grid to study DC grid under various scenarios or its

interaction with AC system. In order to have the same physical characteristics as the high voltage

practical project, this paper presents an equal capacity ratio principle to help the parameter design in

low voltage DC grid. All the parameters are selected according to the parameters of the high voltage

reference system based on equal capacity ratio principle and optimized by simulation model.

Simulation models of original VSC-MTDC and 6-terminal low voltage DC grid are built in

PSCAD/EMTDC to validate the equal capacity ratio principle and the simulation results prove the

equivalency. Based on the voltage margin control, a coordinated master-slave control method is

proposed. The performance of the 6-terminal DC grid is studied under a variety of faults, simulation

results proves that the DC bus voltage of the DC grid can be controlled steadily after faults.

Keywords: DC grid, equal capacity ratio principle, voltage coordinating control, simulation model

INTRODUCTION

Features like high reliability, efficiency, *electromagnetic compatibility and without phase

control requirement or reactive power problems turn

DC grid into an interesting and promising

technological option. The DC grid has superior

characteristics compared with the AC grid. Each

power generator connected to the DC grid can easily

be operated cooperatively because it controls only

the DC bus voltage. With the rapid development of

distributed generation, energy storage systems (ESS)

and power electronic loads, future power systems

will be certainly more and more based on direct

current (DC) architectures. * *This work is supported by National Natural Science

Foundation of China( 51177044), Sino-Danish Strategic

Research Cooperation within Sustainable end Renewable

Energy(2014DFG72620

**Corresponding author: E-mail: [email protected]

Adoption of a DC grid provide more operational

flexibility, such as: increased control over DC and

AC side power flow; active power could be

exchanged while each ac network maintains its

autonomy, hence decreasing the risk of AC fault

propagation from one AC network to another; low

transmission losses; and could optimize the

performance of nearby AC lines in terms of active

and reactive power flow [1-3]. Large offshore wind

farms located far from their grid connection point

will require HVDC to connect to shore to reduce

cable losses and decrease reactive power

requirements [4-5]. Moreover, a DC grid based on

multi-terminal voltage-source converter multi-

terminal direct current (DC) technology (VSC-

MTDC) might offer significant advantages for the

interconnection of the turbines within the wind farm

[6-7]. Practical projects of DC grid have been

Proceedings from The 55th Conference on Simulation and Modelling (SIMS 55), 21-22 October, 2014. Aalborg, Denmark

293

designed in the European countries to connect

offshore wind farms to the AC grids.

Other practical efforts that develop DC grid can be

seen in US, Japan, Korea and European countries

through the efforts of conceptual design and

demonstration projects for DC grid [8]. In order to

study DC grid under various scenarios or its

interaction with AC system, an acceptable solution is

setting up a low voltage, small capacity DC grid in

laboratory. Therefore, this paper designs a 6-terminal

DC grid. The priority objective of the low voltage

DC grid is having the same physical characteristics

as the high voltage practical project. According to

this requirement, this paper presents an equal

capacity ratio principle to help the parameter design

in low voltage DC grid. All the parameters are

selected according to the parameters of the high

voltage reference system based on equal capacity

ratio principle and optimized by simulation model.

CONFIGURATION OF THE DC GRID

The low voltage DC grid in laboratory is designed at

500V with 6 terminals which is given in Figure 1.The

capacity of each terminal is set at 10kW. Terminal 1,

Terminal 2 and Terminal 3 are connected to the AC

system. Terminal 4 is connected to energy storage

system with bidirectional DC/DC as its interface.

Wind power system or a PV system is integrated to

the DC grid at Terminal 5. The last terminal is

designed to provide electric power to AC load

through VSC which using three-phase two-level

topology.

CVT6

CVT4

Load CVT5

CVT2

CVT3

CVT1

Figure 1: Configuration of the DC grid

Figure 2 shows the voltage source converter applied

in DC grid. us is AC power system voltage and uc is

output voltage of the converter at AC side. i is the

converter current and is is the power system current.

upcc1 is the voltage at PCC. udc is voltage of the

converter at DC side and idc is the current inject to the

DC grid by the converter. idc line is the current flow

through the DC line. PS and QS is the power inject to

the AC power system by the converter. PC and QC is

the power flow to the converter. R and L is the

equivalent resistance and inductor between AC power

system and the converter, therefore the transformer in

figure 2 is ideal.

dcu

su cupcc1u

SPSQ

CPCQ dci

R L、

si

filter

dcP

i

,dc linei

Figure 2: The voltage source converter in DC grid

EQUAL CAPACITY RATIO PRINCIPLE

Most of the parameter design method is only suitable

for high voltage with large capacity VSC-MTDC

system. The priority objective of the low voltage DC

grid is having the same physical characteristics as the

high voltage practical project. Therefore, this paper

uses equal capacity ratio principle to help the

parameter design in low voltage DC grid. The

parameters in low voltage DC grid is selected

according to the high voltage reference system at first,

and then optimized by simulation model. 2 2

1 1 1 2 2 2

1 2

N N

N N

CU C U

S S

(1)

2 2

1 1 1 2 2 2

1 2

N N

N N

L I L I

S S

(2)

Where SN1 and SN2 is rated capacity of each system,

UN1 and UN2 is rated voltage of each system, C1 and

C2 is capacitor in each system, L1 and L2 is inductor in

each system, ω1 and ω2 is angular frequency of each

system.

DC CAPACITOR

Parameter design

The reference system is a 10kW DC grid, its DC

voltage is set at 800V, and the capacitor at DC side is

1020μF. By following equation (1) 22

25001020 800

10 10

C

k k

(3)


294

Equation (3) gives the DC capacitor is 2611.2μF, and

its optimized value is 2400μF. As the convertor is

bipolar topology, the grounded capacitor at each polar

is 4800μF.

Verification

The DC Capacitor directly impacts transient

characteristics of DC grid. When there is a fault in

AC system, both the AC and DC system will see

large scale power oscillation which may lead to

overvoltage. The unbalanced fault cause secondary

harmonic oscillation voltage. As the reactance of DC

capacitor correspond to the second harmonic power is

relevant high, it cause bigger voltage oscillation.

Therefore, it is important to consider the

inhibition effects when design the DC capacitor.

Reduce DC voltage oscillation

When a system is unbalance, the second order

harmonic active power is

1 13 cos(2 + )= cos(2 + )S N N NP kU I t kS t

(4)

Where k is second order harmonic active power

oscillating coefficient, SN is the rated capacity of the

system, φ1 is initial phase angle of second order

harmonic power. If we only consider the DC

component and second order voltage harmonic, the

voltage of the converter is

2= sin(2 + )d DC DCu U U t (5)

Where UDC is the DC component, φ2 is initial phase

angle of second order harmonic power,

2sin(2 t+ )DCU is the second order voltage

harmonic. Equation (5) gives the second order

harmonic power at the DC side as

)2cos()2sin(2

)2cos(2

222

2

ttUC

tUUCP

DCd

DCDCdd (6)

As 2△UDC2 in Equation (6) is relevantly small, the

second part in Equation (6) can be ignored. Equation

(6) is rearranged as

22 cos(2 + )d d DC DCP C U U t (7)

Neglect the switching loss and transmission line loss

which gives us △ PS=△ Pd. We also suggest φ1=φ2,

by combination of Equation (5) and Equation (6), we

get

=2

Nd

DC DC

kSC

U U

(8)

Suppose the max value of voltage fluctuation value as

△ UDCmax, therefore

2N

d

DC DCmax

kSC

U U

(9)

Or

2(0<k 1)

2 ( )

Nd

DCmax DC

DC

SkC

U U

U

(10)

While k=1

FCd

24.1273500

10000

05.05022

12

Store electric power

The DC capacitor storage electric power which could

last for a certain time to guarantee the system

operates at rated power. Assume the time constant

related to DC capacitor is τ and it equals to 2

=2d D

N

C U

S

(11)

If τ is smaller than 5ms, the capacitor value in

equation (11) can inhibit small disturbance or

transient overvoltage. Normally τ is 2ms in real

project, therefore -3 3

2 2

2 2 10 2 10 10= =160 =2

500N

d

D

SC F ms

U

（）

-3 3

2 2

2 5 10 2 10 10= =400 =5

500N

d

D

SC F ms

U

（）

All in summary, the DC capacitor value is set at

2400μF.

FILTER PARAMETER DESIGN

Most of the filter in VSC-MTDC is high pass filter,

and second order high pass filter is the most widely

used. Suppose the filter cutoff frequency is 450Hz

and Qfilter is 0.08. According to the equal capacity

ratio principle, the capacity is 0.25kW (each phase).

Figure 3 gives the impedance characteristic of the

filter.

0 200 400 600 800 1000 1200 1400 1600 1800

50

1

00

1

50

2

00

2

50

3

00

Figure 1: The impedance characteristic of the filter


295

DC GRID TRANSMISSION LINE

The transmission line is also chosen according to the

equal capacity ratio principle and use T-type

equivalent circuit. The reference system is a ±200kV

high voltage system with its capacity equals to

200MW. The DC grid in laboratory is designed at

±250V, each terminal capacity is 10kW. Thus the

voltage ratio is 800, power ration is 20000 and

impedance ratio is 32. We get the T-type equivalent

circuit and line parameter as figure 4. 0.07ohm 0.07ohm0.005H 0.005H

0.026uF

Figure 2: T-type equivalent circuit

REACTOR VALUE DESIGN

The output voltage of converter at AC side uc is

1 1=(1+ )C PCC eq PCCU U I X X U

(12)

Where X=ωL.

The relationship between output voltage of converter

at AC side and DC side is

2C DC

MU U

(12)

Where μ is utilization efficiency of DC voltage, M is

the modulation ratio. UDC is DC voltage. Because of

the control margin and fluctuation of AC and DC

voltage, M is set at 0.95. Therefore,

1

0.95(1 )

2C PCC DCU X U U

(14)

The nominal voltage value at DC side is 500V and X

equals to 0.25. Substituted in equation (14),

1

30.95

2(1 0.25) 5002

PCCU ，

1 232.7PCCU V (15)

The low order harmonics increase along with the

Upcc1 and M decrease, therefore the transformer

secondary voltage (phase to phase) is 230V. Define

SaB=10kVA, USB=230V, thus

1025.1

230 3SBI A (16)

Thus

2305.2905

25.1 3aBZ (17)

Therefore the equivalent reactance is 0.25

4.21aBZL mH

(18)

Take off the leakage reactance of the transformer

which is about 1.4032mH, the reactor value is

2.81mH.

EQUIVALENCY VERIFICATION

All the parameters of the 6-terminal low voltage DC

grid is designed based on a 500kV high voltage VSC-

MTDC system which has 3 terminals. The voltage

source converter uses the three-phase two-level

topology. The DC voltage of VSC1 is controlled at

constant which is 500kV, while the power flow

through VSC2 and VSC3 is controlled constant

which are 200MW and -200MW. Simulation models

of original VSC-MTDC and 6-terminal low voltage

DC grid are built in PSCAD/EMTDC to validate the

equal capacity ratio principle. Figure 5 illustrates

voltage waveform of VSC-MTDC system. The

voltage waveform (Terminal 1, Terminal 2 and

Terminal 3) of DC grid is shown in figure 6. The

simulation results of low voltage DC grid mainly

agree with the VSC-MTDC system DC voltage

waveform, which proves the equivalency.

Figure 3: voltage waveform of VSC-MTDC system

Figure 4: Voltage waveform of DC grid

CONTROL SYSTEM

Master-slave control, voltage margin method and

droop control are typical control methods for DC grid.

When the master-slave strategy is employed to

regulate DC bus voltage in a DC grid, its voltage is

determined by the constant DC voltage control

converter. At this scenario, the DC voltage will be


296

unstable when the active power is unbalance or the

constant DC voltage control terminal is tripped off

against faults. Therefore the DC voltage must be

controlled through the coordinating control of the

system supervisor layer through communication.

Based on the research work above, this paper designs

control strategies for DC grid which include control

strategy for VSC and bidirectional DC/DC terminal,

and coordinated control strategy among multiple

terminals.

dcV dcV dcVdcV

P P P P1VSC 2VSC 3VSC 4VSC

2U3U

4U

Figure 5: Operation and control characteristics of the

DC grid

Based on the voltage margin control, a coordinated

master-slave control method is proposed. Figure

shows the operation and control characteristics of the

DC grid. Accordance with the laboratory voltage

level, the voltage margin value is calculated,

whereΔU2<ΔU3<ΔU4. Under the steady state, their

operational characteristics follow the red line. The

DC voltage of VSC1 is controlled at constant, the rest

three converters are designed to deliver or inject

proper active power. If there is a fault at Terminal 1,

the DC voltage at Terminal 2 is controlled at constant,

which means its operational characteristics change to

the green line and other terminals remain unchanged.

The basic principal is the converter with smallest

voltage margin will be considered to control DC

voltage first. Instead of following the red line, the

selected converter will adjust to the green line. The

black lines give the operation limit of each converter.

SIMULATION RESULTS

The model of the 6-terminal low voltage DC grid in

Fig.1 is tested in PSCAD/EMTDC.The DC voltage at

Terminal 1 is controlled at 500V. Terminal 2 delivers

3.5kW active power from AC system to the DC grid,

while Terminal 3 delivers 3.5kW active power from

DC grid to the DC system. Terminal 2 is connected to

energy storage system with bidirectional DC/DC as

its interface. A PV system is integrated to the DC grid

at Terminal 5. The last terminal provides electric

power to AC load through VSC which using three-

phase two-level topology.

Case 1

The output power of Terminal 2 is 3.5kW, it decrease

to 2.5kW at 0.8s, at 1.2s it increase to 4kW and

Figure 8(a) is its simulation result. The output power

of Terminal 3 is -3.5kW, it decreases to -5kW at 0.8s,

at 1.2s it increases to -2.5kW. Figure 8 (b) gives the

simulation result. The DC voltage waveform of

Terminal 1, Terminal 2 and Terminal 3 is shown in

Figure 9. Figure 10 illustrates the AC current

waveform of Terminal 2 and Terminal 3 during the

simulation process.

(a) (b)

Figure 6: Simulation result of output power

(a) Terminal 1

(b) Terminal 2

(c) Terminal 3

Figure 7: The DC voltage waveform

(a) Terminal 2


297

(b) Terminal 3

Figure 8: The AC current waveform

Case 2

Figure 11 gives the simulation results of DC

waveform at each terminal when there is a short

circuit fault at the DC side of Terminal 3 at 1s and

last for 0.05s. Figure 12 is the current waveform.

(a) Terminal 1

(b) Terminal 2

(c) Terminal 3

Figure 9: DC waveform at each terminal

(a) Terminal 1

(b) Terminal 2

(c) Terminal 3

Figure 10: current waveform at each terminal

Figure 13 gives the simulation results of DC

waveform at each terminal when there is a three-

phase short circuit fault at the AC side of Terminal 3

at 1s and last for 0.05s. Figure 14 shows the current

waveform.

(a) Terminal 1

(b) Terminal 2

(c) Terminal 3

Figure 11: DC waveform at each terminal

(a) Terminal 1

(b) Terminal 2


298

(c) Terminal 3

Figure 12: current waveform at each terminal

The simulation results prove that the DC bus voltage

of the DC grid can be controlled steadily after a

variety of faults which include grounding fault at AC

or DC side of the system.

CONCLUSIONS

Developing a low voltage, small capacity DC grid in

laboratory to study DC grid under various scenarios

or its interaction with AC system is convenient and

practical. The equal capacity ratio principle helps the

parameter design in low voltage DC grid. All the

parameters are selected according to the parameters

of the high voltage reference system based on equal

capacity ratio principle and optimized by simulation

model. The simulation results prove that the low

voltage, small capacity DC grid has the same

physical characteristics as the high voltage practical

project.

Based on the voltage margin control, a coordinated

master-slave control method is proposed. Accordance

with the laboratory voltage level, the voltage margin

value is calculated. With carefully selected margins

based on the system strength and converter type, the

DC bus voltage of the DC grid can be controlled

steadily after a variety of faults. A digital simulation

model of the 6-terminal low-voltage DC grid in

laboratory is built using PSCAD/EMTDC.

Simulation results validate the feasibility of the

proposed coordinated control strategy. It can maintain

the DC bus voltage and against active power

unbalance or tripping off converters.

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